1. Field of the Invention
The present invention relates to a gas sensor.
2. Description of Related Art
An example of known gas sensors for detecting a specific gas is an oxygen sensor which includes a cell having a pair of electrodes disposed on the outer surface of a solid electrolyte member and which measures the air-fuel ratio of air-fuel mixture supplied to an internal combustion engine (hereinafter referred to as the air-fuel ratio of the internal combustion engine). Particularly, a laminate-type gas sensor is known in which two cells are stacked or laminated with a measurement chamber formed therebetween, and which is adapted to detect the concentration of oxygen contained in exhaust gas over the full range of the air-fuel ratio of the internal combustion engine (see Patent Document 1 below, etc.). In the laminate-type gas sensor, a first cell (also called an “oxygen-concentration detection cell”) outputs to an external control circuit a voltage corresponding to the concentration of oxygen within the measurement chamber, and a second cell (also called an “oxygen pump cell”) pumps oxygen out of the measurement chamber or pumps oxygen into the measurement chamber in accordance with a current supplied from the control circuit.
As described in Patent Document 1, such a laminate-type gas sensor is usually driven by a control circuit which feedback-controls the current supplied to the oxygen pump cell (hereinafter also referred to as the “pump current”) on the basis of the output voltage of the oxygen-concentration detection cell. However, such a laminate-type gas sensor has a drawback. Namely, when the pump current of the oxygen pump cell is changed, the output voltage of the oxygen-concentration detection cell may fail to follow the change in the pump current and may change in accordance with a time lag. When the time lag becomes excessively large, the control circuit oscillates and the sensor output fluctuates, which makes stable measurement difficult and lowers measurement accuracy.
In order to overcome the above-mentioned drawback, according to the technique described in Patent Document 1, a high pass filter composed of a resistor and a capacitor is provided in the control circuit. This configuration prevents oscillation of the control circuit and prevents the output voltage of the oxygen-concentration detection cell from being greatly affected by a change in the pump current of the oxygen pump cell.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2002-243700
As described above, a dedicated control circuit including a high pass filter composed of a resistor and a capacitor as disclosed in Patent Document 1 has been conventionally used for a laminate-type gas sensor having two cells; i.e., an oxygen pump cell and an oxygen-concentration detection cell. This is because use of such a dedicated control circuit has been considered desirable. However, the use of such a dedicated control circuit may impair compatibility of the control circuit with a gas sensor of a type which does not cause the control circuit to oscillate and which avoids the necessity of providing a high pass filter in the control circuit. Also, since a resistor and a capacitor must be added in order to provide a high pass filter in the control circuit, the manufacturing cost may thereby increase.